I like the game idea very much. I didn't read the description (or title) properly and so I died early. But even without having read it I directly got: the shadow is important. So the game is really intuitive in that regards.
Unfortunately I don't get how to solve the second level. I only can switch over to become another shadow but don't know how to get back. And when you stick your two shadows together they also somehow become immortal ^^. So I had to reload the page in order to restart.
So one thing you could add would be a restart button - or at least having a shortcut for it (like [R] restarts the level).
Overall pretty good idea and I like how you used the theme. I spend a lot of time thinking about how to use it. But that isn't even close to any entry of my list ^^.